XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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Two-layer QM/QMí calculations for a geometry optimization of large biradical systems
Yasutaka Kitagawa, Toru Saito, Yusuke Kataoka, Natsumi Yasuda, Hiroshi Hatake,
Toru Matsui, Takashi Kawakami, Shusuke Yamanaka, Mitsutaka Okumura, Kizashi Yamaguch

Graduate School of Science, Osaka University, Japan
With the recent progress in quantum chemistry, we can calculate electronic structures, energies and energy derivatives of large molecules by the first principle methods. A broken-symmetry (BS) (or an unrestricted: U) method approximately but easily corrects the static correlation at the lower computational costs. However the BS method involves a serious problem called a spin contamination error (SCE). For the problem, our group has proposed a spin-projection method to eliminate the SCE from the energy derivatives based on Yamaguchiís approximate spin projection (AP) procedure [1-4]. By the AP method, one can optimize the geometry of the biradical systems without SCE at the costs of the BS level calculations. However one must carry out 6N (N = optimizing atoms) times single-point calculations previous to the geometry optimization because it uses a numerical derivative for d<S2>/d R values. In addition, it also requires both the low-spin and the high-spin state calculations during the geometry optimization. Therefore the reduction of the computational costs is a problem of the AP method for the optimization of the larger biradical systems such as a binuclear metal complex. In this study, we attempt to combine the AP method and the spin-restricted (R) methods, i.e. two-layer QM/QMí approach based on ONIOM method. In the method, the effect of the outer-ligands is included by the restricted method whilst an energy gradient of the core is calculated by the AP method using a reduced (small) model. The detail about the method and results are illustrated in the presentation.

References
[1] Y. Kitagawa, T. Saito, M. Ito, M. Shoji, K. Koizumi, S. Yamanaka, T. Kawakami, M. Okumura, K. Yamaguchi, Chem. Phys. Lett. 2007, 442], 445.
[2] T. Saito, Y. Kitagawa, M. Shoji, Y. Nakanishi, M. Ito, T. Kawakami, M. Okumura, K. Yamaguchi, {it\ Chem. Phys. Lett
. 2008, 456, 76.
[3] T. Saito, S. Nishihara, Y. Kataoka, Y. Nakanishi, T. Matsui, Y. Kitagawa, T. Kawakami, M. Okumura, K. Yamaguchi, Chem. Phys. Lett., 2009, 483, 168.
[4] Y. Kitagawa, T. Saito, Y. Nakanishi, Y. Kataoka, T. Matsui, T. Kawakami, M. Okumura, K. Yamaguchi, J. Phys. Chem. A., 2009, 113, 15041.


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